The present invention relates to ion-transport membranes of tubular form that are fabricated from ceramics that are capable of separating oxygen or hydrogen from a feed through ion transport. More particularly, the present invention relates to such a membrane that is internally supported by one or more structural members to prevent inward collapse of the membrane under the application of pressure.
There exist ceramic materials that exhibit an ion-transport capability of either oxygen ions or protons at elevated temperatures and upon the application of a differential pressure to opposite surfaces of the material. Ceramic materials that have an oxygen ion-transport capability can be used to fabricate oxygen transport membranes to separate oxygen from an oxygen containing feed. Ceramic materials that possess a proton conducting capability can be used to construct hydrogen transport membranes to separate hydrogen from a hydrogen containing feed. For instance, in oxygen transport membranes, an oxygen containing feed contacts a cathode side of the membrane in which the oxygen is ionized by gaining two electrons. The oxygen ions travel through the ceramic material making up the membrane and emerge from an opposite anode side. At the anode side, the oxygen ions lose the electrons to reconstitute the oxygen molecules.
In mixed conducting ceramic materials, the electrons travel from the anode side to the cathode side of the membrane to ionize the oxygen. In ionic conducting materials, external electrodes are applied to the membrane to provide a return path for the electrons. In dual phase conducting materials, a metallic phase is incorporated into the ceramic material to serve as a return pathway for the electrons. The driving force for the ion-transport is a pressure differential. For instance, in case of an oxygen transport membrane, the oxygen partial pressure at the cathode side of the membrane is greater than that at the anode side. Hydrogen transport membranes function in a similar fashion.
Ion-transport membranes can be formed as an array of ceramic membrane tubes. In case of an oxygen transport membrane array, an oxygen containing feed, for instance, air, is applied, depending on the design of the array, to the inner or outer surfaces of the tubes under pressure. At the same time, the tubes are heated by a variety of known techniques to an elevated operating temperature. The oxygen separated from the oxygen containing feed by the array can be discharged or reacted with an externally fed substance or substances in the presence of a catalyst. For instance, a steam and hydrocarbon containing feed can be introduced to the anode sides of an array of membrane tubes to produce a synthesis gas in the presence of a known steam methane reforming catalyst.
All ceramic materials, including ceramic membrane materials, to some degree or other, tend to exhibit a behavior known as creep. Creep can be defined as permanent plastic deformation due to an applied stress. Creep deformation tends to increase as the applied stress and as operational temperatures increase. Ceramic ion-transport membranes are susceptible to creep because they operate at a temperature above 400xc2x0 C., preferably in a range of between about 500xc2x0 C. and about 1200xc2x0 C. and more commonly, between about 800xc2x0 C. and about 1000xc2x0 C. The pressure applied to the outside of the ceramic membrane tubes produces a stress distribution which, in combination with the elevated temperature, produces creep.
Creep ideally tends to cause a tubular, ion-transport membrane subjected to an external pressure to deform in a manner that produces a reduction in tube diameter and an increase in tube wall thickness. The foregoing is an ideal deformation in that few, if any, tubes will deform in such a manner due to manufacturing imperfections. Such imperfections result in an unsymmetrical stress distribution in the tubes when an external pressure is applied. The uneven stress distribution in turn tends to produce uneven deformations within different portions of the tube that act to accentuate the uneven stress distribution. This mechanism leads to further uneven deformation. Eventually the tube will buckle to assume a squashed, virtually flat appearance. Common manufacturing imperfections in a ceramic membrane tube include: uneven wall thickness; ovality; and lack of material homogeneity.
It has been found that the deformation and failure process due to creep occurs more quickly, the higher the creep rate of the ceramic, the higher the temperature, the greater the pressure differential, and the greater the degree of manufacturing irregularities.
In the prior art, there exist tubular, ion-transport membrane structures that are internally reinforced and therefore, naturally resist creep effects. An example of this can be found in U.S. Pat. No. 5,599,383. In this patent, membranes are disclosed in which a tubular dense mixed conducting oxide layer, that is capable of conducting oxygen ions at elevated temperature, is applied as an outer layer to a solid, porous rod-like structure. As may be appreciated, the thermal expansion characteristics of the central rod-like structure must be the same as or at least very similar to the mixed conducting oxide layer. Differential thermal expansion characteristics will cause breakage of adjoining layers. In this patent, this is not a problem because the central rod-like structure is an active layer that serves to separate oxygen. As such it can be made of the same material as the adjoining mixed conducting oxide layer or at least a material having similar thermal expansion characteristics.
As will be discussed, the present invention provides a structural supporting member for a tubular ceramic membrane. The structural supporting member helps the membrane resist creep and does not have to be formed of a material that has similar thermal expansion characteristics to the tubular membrane being supported. Further advantages of the present invention will become apparent from the following discussion.
The present invention provides an ion-transport membrane assembly that comprises a tubular, ion-transport membrane and a plurality of structural supporting members. The tubular, ion-transport membrane is in the form of a tube and is fabricated from at least one ceramic, ion-transport material capable of ionic transport at an operational temperature of greater than about 400xc2x0 C. Such a material can be a mixed conductor that conducts both ions and electrons or can be an ionic conductor that solely conducts ions. The material can also be a dual phase conductor in which a metallic phase composed of a metallic oxide or a metal is used for electronic conduction. Additionally, the membrane may either be an oxygen transport membrane that serves to separate oxygen or a hydrogen transport membrane that acts to separate hydrogen through conduction of protons.
The plurality of structural supporting members are fabricated from an open porous material that has an outer cylindrical surface. Moreover, the plurality of structural supporting members is inserted within the membrane, without being bonded to the membrane, to inhibit inward collapse of the membrane under application of pressure applied to an exterior surface of the membrane. The plurality of structural supporting members is configured to permit relative movement between the outer cylindrical surface thereof and an interior surface of the tubular, ion-transport membrane when the temperature of the ion-transport membrane assembly increases to and decreases from the operational temperature. Thus, thermal characteristics of the structural supporting member do not have to be matched to the ceramic membrane. For instance, the ceramic membrane might be a perovskite while the structural supporting member could be alumina. Moreover, since the plurality of structural supporting members are simply inserted into the tubular membrane construction, expensive and time consuming fabrication steps involving the application of membrane layers onto the structural supporting member is avoided.
Each of the plurality of structural supporting members is preferably a porous, rod-like cylinder. The tubular, ion-transport membrane can be a composite membrane having a dense layer connected to at least one porous layer. A thin dense layer is routinely used to decrease the resistance to transport of ions and the porous layer provides an active enhanced surface area. There can be one or more porous layers on one side of the dense layer or sandwiching the dense layer. The at least one ion-transport material can be a mixed conductor capable of transporting oxygen ions. Preferably the porous material has a porosity of between about 20% and about 90%. More preferably the porous material has a porosity of between about 40% and about 85%. Preferably the porous material is fabricated from a reticulated foam. The porous material is preferably MgO, Al2O3, CeO2, or ZrO2.